Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section

[Will Deacon <will.deacon@arm.com>]

[+more ppc folks]

On Mon, Feb 18, 2019 at 04:50:12PM +0000, Will Deacon wrote:
> On Wed, Feb 13, 2019 at 10:27:09AM -0800, Linus Torvalds wrote:
> > Note that even if mmiowb() is expensive (and I don't think that's
> > actually even the case on ia64), you can - and probably should - do
> > what PowerPC does.
> > 
> > Doing an IO barrier on PowerPC is insanely expensive, but they solve
> > that simply track the whole "have I done any IO" manually. It's not
> > even that expensive, it just uses a percpu flag.
> > 
> > (Admittedly, PowerPC makes it less obvious that it's a percpu variable
> > because it's actually in the special "paca" region that is like a
> > hyper-local percpu area).

[...]

> > But we *could* first just do the mmiowb() unconditionally in the ia64
> > unlocking code, and then see if anybody notices?
> 
> I'll hack this up as a starting point. We can always try to be clever later
> on if it's deemed necessary.

Ok, so I started hacking this up in core code with the percpu flag (since
riscv apparently needs it), but I've now realised that I don't understand
how the PowerPC trick works after all. Consider the following:

	spin_lock(&foo);	// io_sync = 0
	outb(42, port);		// io_sync = 1
	spin_lock(&bar);	// io_sync = 0
	...
	spin_unlock(&bar);
	spin_unlock(&foo);

The inner lock could even happen in an irq afaict, but we'll end up skipping
the mmiowb()/sync because the io_sync flag is unconditionally cleared by
spin_lock(). Fixing this is complicated by the fact that I/O writes can be
performed in preemptible context with no locks held, so we can end up
spuriously setting the io_sync flag for arbitrary CPUs, hence the desire
to clear it in spin_lock().

If the paca entry was more than a byte, we could probably track that a
spinlock is held and then avoid clearing the flag prematurely, but I have
a feeling that I'm missing something. Anybody know how this is supposed to
work?

Will